1. Field of the Invention
The invention is a short residence coal liquefaction process in which more than fifty percent of the carbon in coal is converted to liquids, while limiting production of hydrocarbon (HC) gases, resulting in high ratios of liquids/HC gases.
2. State of the Art
Structurally, bituminous coal typically consists of monocyclic and condensed aromatic and hydroaromatic rings (clusters), varying in size from a single ring to perhaps four or five rings, which are linked to each other by connecting bridges which are typically short aliphatic chains or etheric linkages. Generally, coal liquefaction processes occur in the temperature range of 400.degree. C.-500.degree. C. by rupturing the connecting bridges to form free radicals or ions. The free radicals or ions are then capped by a small entity such as hydrogen. If the free radicals are not capped, they will combine in condensation or polymerization reactions to produce large structures which will be solid at room temperature.
Prior art coal liquefaction processes can be grouped into four different types of processes: pyrolysis (including hydropyrolysis), solvent extraction, catalytic hydrogenation with a solvent, and Fischer-Tropsch which is an indirect process.
In pyrolysis processes, coal is heated to 400.degree. C. to 500.degree. C. in the absence of any reacting atmosphere or in the case of hydropyrolysis, a hydrogen atmosphere, but without an externally-applied catalyst. The connecting bridges between the condensed ring units are thermally ruptured and the free radicals which are formed are stabilized by capping with hydrogen which is abstracted from some of the structural units in the coal. The total yield of liquids and gases by pyrolysis is typically in the range of 40% by weight of the coal. The remaining 60% by weight of the coal is a solid residue known as char.
Solvent extraction processes typically involve dissolving coal in a hydrogen donor solvent and heating to 400.degree. C. to 450.degree. C. One of the more advanced solvent extraction processes is the Exxon Donor Solvent Process of Exxon Oil Company. In this process a hydrogen donor solvent is added to coal feedstock to form a slurry which is then heated to a temperature of approximately 450.degree. C. for approximately 15-20 minutes. While heating, hydrogen gas is added to the slurry.
In catalytic hydrogenation with a solvent, coal is dissolved in a hydrogen donor solvent, e.g. tetralin, to form a slurry, a hydrogenation catalyst is then introduced into the slurry and the slurry is heated to above 400.degree. C. Hydrogen addition to the coal is approximately 4% to 5% by weight and the product is a liquid and gas (C.sub.1 -C.sub.4 hydrocarbons) at room temperature. One of the most successful examples of a catalytic hydrogenation with a solvent process is the H-Coal process developed by Hydrocarbon Research, Inc.
The Fischer-Tropsch coal liquefaction technology, is the only liquefaction technology that is being utilized on a commercial scale. In the Fischer-Tropsch process, coal is gasified with oxygen and steam at a temperature which is usually above 950.degree. C., to produce carbon monoxide and hydrogen. These gases are then reacted at a temperature of approximately 430.degree. C., in the presence of an appropriate catalyst, to form gaseous and liquid hydrocarbons. In an alternative technology to produce hydrocarbons, coal is gasified to CO and H.sub.2, which are then converted, principally to methanol by well-known technology. The methanol is then converted to gasoline using the Mobil ZSM-5 catalyst.
The prior art direct coal liquefaction technologies produce large amounts of hydrocarbon (HC) gases, ratios of liquids to HC gases usually being of the order 3/1 to 4/1, with none reported greater than about 7/. Residence times of the materials (reactants plus products) in the temperature zone above 350.degree. C. are characteristically between 15 minutes and one hour. Such long exposure of the primary liquid molecules to temperatures above 350.degree. C. results in extensive thermal cracking, yielding hydrocarbon gases. Since more than half of the gases thus formed is methane, this cracking results in large consumption of hydrogen.